Marine biologists and ocean activists have grown alarmed about the seaborne plastic that fouls shorelines and clogs currents from the Arctic to the South Pacific. But the actual amount and source of it hasn’t been known because consumer habits and pollution-control practices vary so widely world-wide.

In a new accounting of global garbage, researchers in the U.S. and Australia led by Jenna Jambeck, an environmental engineer at the University of Georgia, calculated the share that each of 192 countries could have contributed to plastic waste in the oceans. Their study is based on consumer data and waste-management information covering coastal populations around the world. The U.S. ranked 20th by the researchers’ estimates, deemed responsible for just under 1% of the mismanaged plastic waste.

Unchecked, the amount of plastic waste fouling the seas may double by 2025, reaching levels “equal to 10 bags full of plastic per foot of coastline,” Dr. Jambeck said.

According to the researchers, the coastal population of China generated 8.82 million metric tons of mismanaged plastic waste in 2010, about 27.7% of the world total. Of that, between 1.32 million and 3.53 million metric tons ended up as marine debris.